1. Field of the Invention
This disclosure relates to integrated circuit fabrication, and more particularly to a system and method for conditioning a polishing pad used in a chemical-mechanical polishing process.
2. Description of the Related Art
Modern integrated circuits (ICs) employ advanced transistor isolation and multi-level interconnect techniques to increase both circuit functionality and processing speed. Conventional transistor isolation fabrication techniques utilizing LOCOS (LOcal Oxidation of Silicon) have been virtually superceded by STI (Shallow Trench Isolation) technology to overcome the xe2x80x9cbird""s beakxe2x80x9d effect associated with LOCOS processing and allows for increased device packing densities. Multi-layer interconnects are pervasively used to facilitate interconnect routing between the transistors of the IC devices, also enabling increased packing densities. In addition, copper interconnects are being implemented in place of conventional aluminum interconnects, due to their improved conductivity and resistance to electromigration over aluminum, to reduce interconnect routing delays and thereby improve processing speed.
STI processing involves the formation of trenches recessed into a semiconductor substrate between adjacent active regions of the IC device. The trenches are then filled in with a dielectric material and subsequently planarized so that the uppermost surfaces of the dielectric and the substrate are approximately equal. Common dielectric materials include oxides, nitrides, or oxynitrides. Interconnect processing involves the formation of an interlevel dielectric between a lower level and an upper level. Contact areas, or vias, are then opened through the interlevel dielectric and subsequently filled in with a conductive material to electrically link the two levels together in the desired interconnect routing scheme. For metallization technologies using inlaid techniques, such as copper interconnects, the metal layers are also created by forming trenches into the interlevel dielectric and filling in the trenches with a conductive material. Additional levels of interconnects may be constructed in the same manner upon the prior levels to form a multi-level interconnect IC device. The interlevel dielectrics are frequently planarized prior to formation of the vias or trenches to minimize elevational disparity across the semiconductor substrate. This facilitates both photolithography of the vias and trenches and provides optimum step coverage of the conductive material being filled in. The conductive layers may also be planarized to form the final interconnect structures.
Modern IC devices simultaneously employ the use of STI and multi-level interconnect technologies to meet the demands for increased functionality and faster processing speeds. Accordingly, planarization of the interlevel dielectrics, conductive layers, and the trench dielectrics is required for optimum fabrication results. Planarization of these layers may be achieved through chemical-mechanical polishing (CMP) techniques, which has received widespread acceptance in the semiconductor processing industry. Generally speaking, CMP processes may be used to globally planarize and remove surface topography irregularities of a material layer(s) through chemical reaction and mechanical abrasion. A typical CMP process involves placing a semiconductor substrate face-down on a polishing pad which is attached to a rotatable table, or platen. An abrasive fluid, known as slurry, is introduced onto the surface of the rotating polishing pad and the substrate is then pressed against the polishing surface by a downward force. The substrate may also be rotated in conjunction with the rotating polishing pad. The chemical-mechanical interaction is provided by solution chemistry and abrasives contained in the slurry. Typical abrasives used by CMP processes include silica, alumina, and ceria. Other abrasives may be utilized and are often matched with the material layer(s) to be removed. Chemical interaction between the slurry and the material layer(s) being polished initiates the polishing process. The abrasives, coupled with the rotational movement of the polishing pad, physically strip the reacted surface material from the substrate. The process continues until the desired thickness amount of the material layer(s) is removed. Upon completion of the polishing process, the substrate is then subjected to a cleaning process to remove residual slurry and foreign particulates, including polish by-products, that may remain on the substrate surface.
By semiconductor fabrication standards, CMP is inherently a dirty process. The use of slurry to facilitate removal of the material layer introduces a significant amount of particles to the substrate surface, which must be removed in the subsequent cleaning step. In addition, during planarization by a CMP process, the substrate surface may be subjected to extremely high local mechanical pressures and exposed to either highly acidic or caustic solutions. Therefore, a substrate planarized by CMP may result in many unwanted defects on or within the upper surfaces. These defects may include, for example, residual particles from the slurry or the abraded substrate surface, chemical contamination from the slurry and/or other fluids, and physical surface damage such as microscratches or film fractures from the mechanical force being applied during polish. These defects have the potential to become yield-limiting defects, affecting die yields of the finished IC devices. For example, microscratches may scratch the surfaces of active regions thereby resulting in higher transistor leakage currents due to crystallographic damages. In addition, a microscratch formed in the surface of the dielectric layer may result in a residual conductive material being trapped into the divots formed by the microscratches during CMP, and potentially short out desired interconnect features. Moreover, residual surface particles may affect areas on the substrate where subsequent photolithography processes occur. The presence of the particles may prevent proper formation of the features defined by the photolithography process. As a result, efforts to substantially reduce the defects introduced by CMP have received considerable awareness.
Efforts to remove residual particles from the polish due to CMP processing have included scrubbing the substrate with brushes, spraying the substrate surface with a pressurized flow of cleaning liquids, and acoustically removing the particles through ultrasonic or megasonic cleaning techniques. Reduction of microscratches have examined minimizing the down force applied to the substrate onto the polishing pad, employing slurries with smaller abrasive grain sizes, and reducing the abrasiveness of the polishing pad. Varying degrees of effectiveness have been gained by these described methods.
Despite the above-described efforts, CMP-induced defects may still be formed and potentially impact final device yields. Considering that CMP processes account for an increasing portion of the entire IC fabrication process flow (STI, local interconnect, inlaid vias and metal), the compounding rate of defects introduced by CMP processes may significantly influence final yields of the IC devices. It would therefore be desirable to provide a method and system for minimizing defects associated with CMW processes. A reduction in defect density of CMP-induced defects may translate into increased die yields of the IC devices being fabricated.
The problems outlined above are in large part addressed by a CMP pad conditioning system and method in which a chemical reagent may be introduced onto the polishing pad during conditioning of the polishing pad. In addition, the chemical reagent may further be introduced onto a storage apparatus that may be used to store the conditioning device and may further be introduced onto the conditioning surface of the conditioning device which is in abrasive contact with the polishing pad during pad conditioning. Introduction of the chemical reagent may reduce the accumulation of previously used slurry (hereinafter xe2x80x9cslurry buildupxe2x80x9d) and glaze present on the polishing pad, on the storage apparatus, and on the conditioning surface. The reduction in slurry buildup and glaze may minimize the formation of defects on the substrate being polished. These defects may include, for example, residual particles and microscratches. The reduction may also minimize reduced polishing rates and increased polishing non-uniformity. A rinsing fluid may also be introduced onto the polishing pad, the storage apparatus, and the conditioning surface to rinse away the accumulated glaze and slurry buildup.
Of the various performance parameters associated with CMP processing, there are two parameters which largely affect optimum CMP results. These parameters are polishing rate and polishing non-uniformity. Polishing rate, typically measured in units of angstroms/minute, is the rate at which the film thickness of the desired material layer is removed. Higher polish rates lead to shorten process times and are often desirable to reduce fabrication cycle times. However, higher polish rates result in increased process control difficulties. Polishing non-uniformity, typically given as a percentage, is the degree of non-planarity of the upper layer surface of the substrate upon completion of the CMP process. Ideal CMP processes exhibit 100% planarity and therefore, the non-uniformnity would be 0%. In reality, however, some degree of non-uniformity will be present and therefore minimizing the degree of polishing non-uniformity is desirable. Factors that may affect the polishing rate and polishing non-uniformity are numerous, and include, for example, slurry composition, applied down force, pad materials, pad rotational speed and slurry flow rate onto the polishing pad.
Polishing pads play an important role in optimum CMP performance. They provide mechanical abrasion for physically removing the material layer from the substrate surface. Polishing pad structure and material properties strongly influence polishing rate and polishing non-uniformity. In general, surface roughness and porosity of the polishing pad determine slurry transport to the substrate surface, material transport away from the substrate surface, and the contact area of the pad to the substrate surface. Therefore, maintaining optimum pad surface roughness and porosity over the useful life of the pad is essential to obtaining ideal polish results. In addition, extending the useful life of the pad is also desirable to minimize costs associated with CMP processes. By extending the pad life, pad changes may occur less frequently and thus reduce the cost of pad consumables. Unfortunately, polishing on the same polishing pad over an extended period induces an undesirable effect known as xe2x80x9cpad glazingxe2x80x9d. Pad glazing results when polishing by-products along with the abrasives in the slurry accumulate on the upper surfaces of the polishing pad, forming a glaze. The glaze smoothes the upper surface of the polishing pad thereby reducing the abrasive properties of the polishing pad. As a result, a reduction in the polishing rate is experienced. In addition, the glazed layers are often unevenly distributed over a polishing pad surface, resulting in localized differences in polishing rates. This results in increased polishing non-uniformity.
In order to minimize the glazing effect, a technique known as pad conditioning may be used to maintain the surface roughness and porosity of the polishing pad. The technique involves mechanically abrading the pad surface in order to remove the glaze and xe2x80x9crenewxe2x80x9d the pad surface. Renewing the pad surface may be accomplished by a conditioning device with a conditioning surface. The conditioning surface may include an abrasive surface to provide the mechanical abrasion. During pad conditioning, the conditioning device is positioned over the polishing pad and a downward force may be applied such that the conditioning surface is in abrasive contact with the polishing pad surface. The conditioning device may sweep back and forth across the polishing pad, which may be continuously rotated, to facilitate removal of the glaze across the entire lateral surface of the polishing pad. The device may also move in a lateral direction from an inner portion to an outer portion of the polishing pad. A rinsing fluid may be continuously injected onto the pad to aid in removing the abraded glaze from the pad surface.
Unfortunately, the mechanical abrasion provided by the conditioning device may not be sufficient to remove the glaze or may fail to remove the glaze from various areas of the polishing pad. In addition, the slurry buildup that may be present on the polishing pad may also fail to be removed. In addition, the glaze and slurry buildup may also be transferred to the conditioning surface of the conditioning device and accumulate on that surface. Upon completion of pad conditioning, the conditioning device is typically positioned away from the polishing pad surface and returned to a storage position. The glaze and slurry buildup may also begin to accumulate on a storage apparatus that may be used to store the conditioning device at the storage position. In a subsequent pad conditioning process, the conditioning device moves from the storage position and is positioned over the polishing pad surface to begin pad conditioning as previously mentioned. The accumulated glaze and slurry buildup on the conditioning surface and/or the storage apparatus may then be transferred back onto the polishing pad. The transferred glaze and slurry buildup may then negatively form defects such as residual particles and microscratches during a CMP process. Prevention or minimization of accumulated glaze and slurry buildup on the polishing pad surface, the conditioning surface, and the storage apparatus is therefore desirable to minimize defect formation, reduced polishing rates, and increased polishing non-uniformity. By introducing a chemical reagent onto the polishing pad, the conditioning surface, and/or the storage apparatus of the conditioning device, accumulated glaze and slurry buildup may be significantly reduced and thus minimize the above-mentioned undesirable effects.
In embodiments of the method and system recited herein, a conditioning device including a conditioning surface may be used to condition a polishing pad. The conditioning surface may preferably contain an abrasive surface including periodic protrusions that extend partially into the polishing pad surface during conditioning. The conditioning surface may be operated to abrade the surface of the polishing pad in order to remove the buildup of slurry and glaze that may be present on the polishing pad surface. During pad conditioning, the conditioning device is positioned over the polishing pad and a downward force may be applied. The downward force is applied such that the conditioning surface may be in abrasive contact with the surface of the polishing pad. To facilitate removal of the glaze and slurry buildup, the polishing pad may be continuously rotated during the pad conditioning process. The conditioning surface may further be rotated in the same direction or in the opposite direction of the rotating polishing pad. The conditioning surface may also be swept back and forth along the polishing pad to provide lateral coverage of the polishing pad. The conditioning surface may also be moved from an outer portion of the polishing pad to an inner portion of the polishing pad to facilitate lateral coverage. To remove the abraded glaze and slurry from the polishing pad, a rinse fluid, preferably deionized water, may be continuously injected onto the polishing pad during the pad conditioning process. The fluid aids to rinse away the abraded glaze and slurry from the polishing pad surface and may be disposed of by a drain residing below the polishing pad to receive the excess of fluids and material wastes generated during the conditioning process.
In the embodiments of the method and system recited herein, the polishing pad may be conditioned with a conditioning device for a predetermined pad conditioning time interval. In one embodiment, the duration of the pad conditioning time interval may be between approximately 20 seconds and approximately 60 seconds and preferably be about 45 seconds. In other embodiments, the duration may be less than approximately 20 seconds or more than approximately 60 seconds. During the predetermined pad conditioning time interval, a chemical reagent may be introduced onto the surface of the polishing pad. The chemical reagent operates to breakup the glaze and slurry buildup and therefore aids in their removal by the abrasive force provided by the conditioning device. The chemical reagent may be introduced for a duration of approximately 20 seconds. In other embodiments, the chemical reagent may be introduced for an entirety of the predetermined pad conditioning time interval, for a duration of more than approximately 20 seconds, or for a duration of less than approximately 20 seconds.
The chemical reagent may further be introduced onto a storage apparatus of the conditioning device while the conditioning device is conditioning the polishing pad. The chemical reagent also serves to breakup any glaze and slurry buildup that may be present on the storage apparatus and therefore prevent their transfer back to the polishing pad surface upon a subsequent pad conditioning process. In addition, when the conditioning device returns to the storage apparatus upon completion of conditioning the polishing pad, the chemical reagent present in the storage apparatus may be in fluid contact with the conditioning surface and serves to breakup any glaze and slurry buildup that may have accumulated on the conditioning surface as well. In one embodiment, the chemical reagent may be introduced onto the storage apparatus concurrently with the chemical reagent being introduced onto the polishing pad during the pad conditioning time interval. The chemical reagent may be introduced onto the storage apparatus for a duration of approximately 20 seconds. Alternatively, the chemical reagent may be introduced for an entirety of the predetermined pad conditioning time interval, for a duration of more than approximately 20 seconds, or for a duration of less than approximately 20 seconds. The chemical reagent may also continue to be introduced onto the storage apparatus while the conditioning device remains at the storage apparatus. A rinse fluid, preferably deionized water, may also be flowed onto the storage apparatus to rinse away the accumulated glaze and slurry buildup on the storage apparatus and the conditioning surface. The fluid may be operated to provide a continuous flow onto the storage apparatus to provide sufficient agitation to remove the accumulated glaze and slurry buildup from the storage apparatus and the conditioning surface. The removed glaze and slurry buildup may then be disposed of by a drain residing below the storage apparatus to receive the excess of fluids and material wastes.
For embodiments in which the conditioning device is not stored onto a storage apparatus but rather suspended in storage position such that the conditioning surface is exposed to the ambient environment, the chemical reagent may be introduced onto the conditioning surface to remove the accumulated glaze and slurry buildup. A rinsing fluid may also be injected onto the conditioning surface to further remove the accumulated glaze and slurry buildup. The fluids and material wastes may then be disposed of by a drain residing below the conditioning device. In addition, the embodiments described herein in regards to sequence and duration of the chemical reagent and rinsing fluid being introduced are understood to be equally applicable to the embodiments in which a storage apparatus is not present.
In the embodiments of the method and system recited herein, a chemical reagent may be introduced to aid in the breakup of accumulated glaze and slurry buildup present on the polishing pad, the storage apparatus, and the conditioning surface. The chemical reagent may preferably be introduced onto both the polishing pad and the storage apparatus during pad conditioning. In an alternative embodiment, the chemical reagent may be introduced onto the storage apparatus during pad conditioning. Although the composition of the chemical reagent being introduced onto the polishing pad, the storage apparatus and the conditioning surface is preferably equal, in alternative embodiments the compositions may be dissimilar. For example, the composition of the chemical reagent introduced onto the polishing pad may be a stronger concentration than the composition of the chemical reagent being introduced onto the storage apparatus or conditioning surface. The chemical reagent may be selected to have a pH approximately equal to the pH of the slurry used in the CUT process. In a more particular embodiment, the pH of the chemical reagent may be between approximately 10 and 11. By way of example, in another particular embodiment, the chemical reagent may include ammonium hydroxide. Ammonium hydroxide has been experimentally observed to be effective a breaking up accumulated glaze and slurry buildup associated with CMP processes. In the particular embodiment, the ammonium hydroxide may be about 2% by volume. In alternative embodiments, the ammonium hydroxide may be greater than about 2% or less than about 2% by volume. In addition, the flow rates used to introduce the chemical reagent onto the polishing pad and storage apparatus may vary. The flow rate used to introduce the chemical reagent onto the storage apparatus may be between approximately 175 ml/min and approximately 225 ml/min. The flow rate used to introduce the chemical reagent onto the polishing pad may be between approximately 600 ml/min and approximately 700 ml/min. For larger or smaller polishing pad diameters and for longer or shorter durations of the pad conditioning time interval, the flow rates of the chemical reagent may be adjusted correspondingly.
A system for conditioning a polishing pad used in a CMP process is also contemplated. A conditioning device may be provided for conditioning the polishing pad. The conditioning device may include a conditioning surface that is operated to be in abrasive contact with the polishing pad during pad conditioning. In one embodiment, the system may include a first conduit for introducing a first chemical reagent onto the conditioning surface. In another embodiment, the system may also include a second conduit for introducing a second chemical reagent onto polishing pad. The system may also include a third conduit for introducing a rinsing fluid onto the conditioning surface. In one embodiment, the composition of the first and second chemical reagents may be equal. In one embodiment, the conduits may be fixtures external to the conditioning device. In another embodiment, the conduits may be fixtures integrated into the conditioning device.
Yet another system for conditioning a polishing pad used in a CMP process is also contemplated. A conditioning device may be provided for conditioning the polishing pad. A storage apparatus for storing the conditioning device may also be included. In one embodiment, the system may include a first conduit for introducing a first chemical reagent onto the storage apparatus. In another embodiment, the system may include a second conduit for introducing a second chemical reagent onto the polishing pad. The system may further include a third conduit for introducing a rinsing fluid onto the storage apparatus. In one embodiment, the composition of the first and second chemical reagents may be equal. In one embodiment, the conduits may be fixtures external to the conditioning device. In another embodiment, the conduits may be fixtures integrated into the conditioning device.